1. Field of the Invention
The invention relates to a method for coupling an electromechanical transducer of a hearing aid or a tinnitus masker to a middle ear ossicle of a hearing impaired person.
2. Description of Related Art
Disorders of the inner or middle ear comprise the most common reason for impairments of hearing. In addition thereto, an increasing number of patients complain about a ringing, whistling or buzzing noise in the head which has no external source, a condition known as tinnitus. In recent years, it has been found that tinnitus may be alleviated or even overcome by providing the patient with a noise signal, for example white narrow band noise, which masks the noise caused by tinnitus.
One approach to overcome the above problems is to provide the patient with a hearing aid and/or tinnitus masker that is fixed to the external ear. However, this approach has several fundamental disadvantages, amongst which are to be named: (1) stigmatization of the patient; (2) the sound is often found to be unsatisfactory due to the limited frequency range and undesired distortion; (3) in many patients the ear canal fitting device leads to an occlusion effect; (4) acoustic feedback when amplification is high.
In view of the above disadvantages of such external hearing aids, John M. Fredrickson et al proposed in "Ongoing Investigations Into An Implantable Electromagnetic Hearing Aid For Moderate To Severe Sensorineural Hearing Loss", Otolaryngologic Clinics Of North America, volume 28, No. 1, February 1995, an implantable middle ear transducer which comprises an electromagnetic motor housed in a hermetically sealed case, where the motor drives a biocompatible elongated probe having a probe tip which is placed in a laser ablated hole in the body of the incus.
In the process of implanting the transducer as described in the above journal article for implantation of the said transducer in the rhesus monkey, a postauricular incision approximately 2 cm behind the external auditory canal is made. The temporalis muscle is transected posteriorly and is reflected anteriorly. An 8.5 mm trephine is used to define the mastoid landmark for an atticotomy approach designed to expose the body of the incus. During the atticotomy, the posterior wall of the bony external auditory canal is thinned, the middle fossa dural plate is exposed, and the incus and incudomalleolar joint are identified. Then a second hole is drilled anteriorly at the root of the zygoma to facilitate probe tip-incus coupling during implantation.
Another partially implantable hearing aid system by Symphonix Devices, Inc., USA uses a "floating mass transducer" (FMT) for directly driving the ossicular chain of a hearing impaired patient with a sensorineural hearing loss (G. R. Ball et al.: Implantable and external hearing systems having a floating mass transducer, U.S. Pat. No. 5,624,376). The FMT is attached to the long process of the incus with a titanium clip using special surgical instruments. The dimensions of the FMT are 1.8 mm in diameter and 2.3 mm in length. After a mastoidectomy, a posterior tympanotomy has to be performed through the facial nerve recess to introduce the FMT into the middle ear cavity and to fix it to the incus (Th. Lenarz et al in "Vibrant Soundbridge System: Ein neuartiges Horimplantat fur Innenohrschwerhorige, Teil 1: Funktionsweise und erste klinische Erfahrungen", Laryngo-Rhino-Otol. 77(1998), 247-255). This procedure inheres a relatively great risk of injuring the adjacent facial nerve.
Both methods described above of directly coupling an implantable hearing aid transducer to the ossicular chain involve an extensive amount of invasive measures with risks of injury of the middle ear, inner ear, and especially the facial nerve.